Implantable bypass grafts are well known for reestablishing blood flow so as to avoid damaged, diseased or constricted blood vessels. One particular situation where bypass grafts are employed is to establish an artery to vein bypass. Such a bypass is established by the use of an elongate tubular bypass graft typically formed of PTFE. In certain instances, the bypass is established in a first procedure by attaching one end of the graft to the side of the brachial artery. The brachial vein is then cut in half. The inflow side of the brachial vein is sewn off and occluded. The outflow side of the brachial vein is connected end-to-end with the bypass graft.
One advantage of this first procedure employing occlusion of the brachial vein is that the bypass graft has a desirable life expectancy. One significant drawback, however, with this first procedure is that after the bypass is established a physician cannot easily distinguish between arterial and venous flows. A contrast injection is typically used by a physician to visually differentiate between arterial and venous flows. Because this first procedure occludes the venous return or inflow side of the brachial vein, both the venous and arterial sides of the bypass simultaneously fill with the contrast injection. Thus, it appears to the physician that the brachial vein has arterial flow and not venous flow, and a physician cannot properly characterize the relative amounts of arterial and venous flows.
A more common, second procedure is to attach one end of the graft to the side of the brachial artery. The graft is then formed into a U-shaped loop, the other end of the graft is then attached to the side of the brachial vein. In this second procedure the physician can more readily visualize the site because the venous flow after the bypass is a combination of arterial flow, which is rich in contrast injection, and venous flow, which is deficient in contrast injection. A physician can, thus, characterize the relative nature of arterial and venous flows with the use of this second procedure.
A disadvantage, however, of this second procedure is reduced life expectancy of the bypass graft as compared to the first procedure. In the second procedure the graft typically occludes in a quicker fashion, as compared to the first procedure, because of increased tissue response of the brachial vein at a location proximal to the bypass graft. Increased tissue response typically results at venous regions that are subject to significant contact with arterial blood flow because arterial blood has greater nutrient content as compared to venous blood.
The increased flow to the venous side brought on by the bypass graft causes an increase tissue response. The increase in tissue buildup may result in undesirable occlusion. In the first procedure where the bypass graft is connected end-to-end, the occlusion caused by tissue buildup is not significantly seen. This is believed to be due to the fact that the flow from the arterial side does not come in direct transverse contact against the venous wall because the flow is directed towards the center of the venous vessel. Where the graft and the vessel are connected end-to-end, a high rate of flow is maintained and an increased tissue response is not seen.
As compared to the first procedure, the second procedure has greater tissue response because flow from the arterial side does come in direct transverse contact against the venous wall. Such direct transverse contact is a consequence of a lack of an end-to-end connection between the graft and the vein and the increased pressure at the graft. Increased pressure is results because the blood pressure on the arterial side is significantly higher than that on the venous side.
Where occlusion does occur within a bypass graft, reintervention in the form of a balloon angioplasty can be employed until such time as the graft fully occludes. At that stage the graft will be relocated to another vein/artery.
It is therefore desirable to provide a bypass graft which may be implanted by the physician with full visualization but once implanted will not result in a significant increase in tissue response which may allude to vessel occlusion.